In low-voltage low power flash memories, for example, where Vcc is between 1.65 and 1.95 V, there is a need for a highly precise voltage reference (Vref) source. This circuit is needed to calibrate the different on-chip power supplies required for operation of the memory. Usually Vref sources are bandgap reference voltage sources based on the compensated behavior of the P-N silicon diode junction. There are a number of bandgap reference voltage sources described in the literature. Many of these circuits offer good stability versus Vcc power supply, temperature range, and process parameter spread.
Unfortunately, precision bandgap reference voltage sources working at low Vcc voltages, such as Vcc of about 1.8 V, typically require a significant amount of current to operate. This current can be in the range of hundreds of microamps, which is too high for flash memories used in portable devices, such as cellular phones. The bandgap circuits also cannot be shut down in “power down” or in standby mode, otherwise latency would be too great when reading the memory in the power down or standby mode. While voltage reference circuits requiring only a few microamps are available, these circuits do not provide the stability required over a range of Vcc power supply voltages, temperatures and process parameter spreads.
For the reasons stated above and for additional reasons stated hereinafter, which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a low current Vref source that has high stability over a range of Vcc power supply voltages, temperatures, and process parameter spreads. The above-mentioned shortcomings of traditional Vref sources and other problems are addressed by the present invention, at least in part, and will be understood by reading and studying the following specification.
Although, various embodiments have been illustrated using particular electronic components it will be understood by those of ordinary skill in the art that other circuit elements could be used to implement the invention and that the present invention is not limited to the arrangement of circuit elements disclosed. Moreover, it will also be understood in the art that the present invention could be applied to a Vref source circuit for use in devices other than flash memory circuits that operate on very low supply voltages. Therefore, the present invention is not limited to a Vref source circuit for very low voltage flash memory.